In the packaging of certain types of foodstuff products such as snack foods like potato and corn chips, cookies and the like, it is common practice to employ a polymeric packaging film for such purposes. A highly desirable property for such packaging films is opacity which protects the packaged foodstuff product from deterioration caused by exposure to light. In particular, it has been found that certain wavelengths of light up to about 450 nm cause increased spoilage in the packaged products. Even when a degree of opacity is present in the film, spoilage may occur if the film allows passage of some light. Consequently, highly opaque films are the most desirable for these purposes.
Typically, such opaque polymeric packaging films are multi-layer films which comprise an opaque, thermoplastic polymeric core layer having one or more skin layers thereon. The skin layers contribute various desirable characteristics to the packaging film such as heat sealability, improved appearance, enhanced machine handling capabilities and the like.
One type of such multilayer opaque packaging films is disclosed in U.S. Pat. No. 4,632,869 to Park et al. Disclosed therein is an opaque, biaxially oriented film structure having a polymer matrix with a strata of cavitated voids, the voids containing spherical void-initiating particles of polybutylene terephthalate. The disclosed structure may also include thermoplastic skin layers and the film can include from about 1% to 3% by weight of a pigment such as TiO.sub.2 or colored oxides.
Another type of opaque packaging film is disclosed in U.S. Pat. No. 4,758,462 to Park et al. This patent also discloses an opaque, biaxially oriented film with a cavitated core layer and transparent skin layers. Colored light absorbing pigments such as carbon black or iron oxide are added to the core and/or the skins in an amount of about 2 to 12 weight % to decrease light transmission through the film.
In addition, U.S. Pat. No. 4,652,489 to Crass et al. discloses an oriented, sealable, opaque polyolefin multi-layer film with a core layer containing vacuoles or voids, a sealable surface layer, and a non-sealable surface layer which incorporates a slip agent such as a polydiorganosiloxane. Further, U.S. Pat. No. 4,944,990 to Liu et al. discloses a multi-layer heat sealable structure for packaging purposes comprising a substrate including a white opaque polyolefin film, which preferably is cavitated. A second layer is coextruded on at least one surface of the substrate, the layer including a heat sealable homopolymer, copolymer, terpolymer or blend thereof. A primer coating containing a vinylidene chloride copolymer is disposed on at least one surface of the second layer and a heat sealable layer is disposed on the primer coating. All of the above-mentioned patents are incorporated by reference herein in their entireties.
Various methods for making a core layer for such polymeric packaging films which has the desired opacity are known in the art. For example, U.S. Pat. No. 4,377,616 to Ashcraft et al. discloses an opaque biaxially oriented polymer film structure which comprises a thermoplastic polymer matrix core possessing numerous voids, a substantial number of which contain at least one spherical void-initiating particle, and transparent thermoplastic skin layers adhering to the surfaces of the core layer. In accordance with the teachings of the patent, the core layer of the film is prepared by melting a mixture of a major proportion of a film-forming polymer such as polypropylene and a minor proportion of incompatible solid filler particles such as polymer which has a higher melting point, at a temperature sufficient to melt the film-forming polymer and to disperse the incompatible particles within the film-forming polymer, extruding this mixture into a film, and then biaxially orienting the extruded film.
In the formation of the film according to the Ashcraft et al. patent, the dispersed incompatible solid filler particles provides sites for the formation of voids surrounding the particles. The resultant opacity of the extruded film results from the fact that, owing to the filler particle content and biaxial stretching, a great number of voids are created. In particular, during the stretching operation, the polymer matrix is torn open at the grain boundaries of the inorganic or organic filler particles which are incompatible with the polyolefin, such that the above indicated voids, that is, empty or unfilled spaces, are formed and the resultant voids or micro-cavities are distributed throughout the volume of the film.
It is taught that these void-initiating filler particles can be organic or inorganic so long as they are spherical in shape and are within a preferred particle size range so as to initiate the formation of a void. Examples of suitable organic particles include thermoplastic resins such as polyamides, certain polyesters such as polyethylene terephthalate and polybutylene terephthalate (hereinafter PBT), and acrylic resins. Inorganic materials suitable for the filler particles include solid or hollow preformed glass spheres, metal beads or spheres, ceramic spheres, and the like.
With this film structure containing distributed voids according to the Ashcraft et al. patent, light passing through the film is refracted or scattered at the voids and therefore imparts the opaque appearance to the film. Thus, these voids provide opacity and also give the film an attractive pearlescent sheen. The structure of the core layer imparts a much higher degree of opacity, possibly due to the effects of light scattering, than that possible from the use of an opacifying pigment alone. In addition, U.S. Pat. No. 5,134,173 to Joesten et al. discloses a method for preparation of an opaque polymeric film having a matrix of a thermoplastic polymer matrix material such as polypropylene having a stratum of voids. Positioned within each of at least a substantial number of the voids is at least one spherical void-initiating solid particle of cross-linked polystyrene, the particle being phase distinct and incompatible with the polymeric matrix material. In preparing the film, a combination of the matrix material and the cross-linked polystyrene particles dispersed therein is extruded into a film structure which is then biaxially oriented either simultaneously or sequentially. The latter two patents also are incorporated by reference herein in their entireties. Further, it is known to prepare opaque polypropylene films using non-spherical inorganic particles of calcium carbonate as the void-initiating filler.
While the above processes for producing opaque polymeric films are generally satisfactory and more than acceptable films are realized therefrom, there may be several problems or disadvantages associated with the processes. For one, the raw material costs of the void-initiating filler particles oftentimes are quite high relative to the raw material costs for the thermoplastic polymeric matrix material. For example, filler particles of PBT, a material which has found wide commercial acceptance for initiating voids in matrix materials of polyolefins such as polypropylene, are quite expensive on a weight basis as compared with the matrix material, oftentimes up to three times more costly. Furthermore, since the filler particles of materials such as PBT have a density greater than the matrix material such as polypropylene, the cost penalty per unit volume or area of the resultant film is even greater. For particles of materials other than PBT, the cost differences may be greater or less depending upon the particular material utilized.
In addition, some materials such as PBT when used as the filler particles may tend to decompose during extrusion of the film, which thereby may leave an objectionable scale or deposit on surfaces of processing equipment. In the case of the decomposition of PBT, this deposit may be manifested in the formation of an eggshell-like scale on the processing equipment. This scale occasionally breaks up, causing visual defects or breaks in the film in downstream processing. For example, it has been found that a typical line for forming white opaque film using PBT as the void-initiating particles can suffer ten or more hours of downtime per month due to these scale formation problems and resultant necessary die cleanups and the like.